For the low pressure polymerization of olefins, it is known to use catalyst systems comprising a transition metal compound and an organometallic compound.
It is also known from Belgian Pat. No. 650,679 to use, as the transition metal compound of the catalyst systems mentioned above, a solid obtained by reacting a transition metal compound with a divalent metal hydroxychloride such as Mg(OH)Cl.
Catalyst systems are also described in Belgian Pat. No. 730,068, in which one of the constituents is also the product of the reaction between a transition metal compound and a solid support consisting of a hydroxylic and/or hydrated divalent metal halide.
Catalyst systems of the same species are described in French patent application Ser. No. 2,014,887, wherein the product of the reaction of a halogen compound of a transition metal with a divalent metal halide complexed by means of an electron donor is employed.
The catalyst systems described in the patents mentioned above are more active than the catalyst systems wherein the transition metal compound is employed directly. However, their activity is still inadequate because the polyolefins produced by means of these systems possess concentrations of catalyst residues which are too high for certain applications.